Head-worn displays (HWD) typically employ a microdisplay on which a two-dimensional (2D) regular image is displayed. Since the physical distance between the microdisplay and the eye is typically much smaller than 25 cm (the closest distance at which the human eye can normally focus), a blurred image forms on the retina unless relay optics are placed in between. The relay optics typically consist of several lenses which serve to form a magnified virtual image of the microdisplay beyond 25 cm (mostly at infinity) on which the eye can then focus and form a sharp retinal image.
Lightweight HWD designs that employ microdisplays (those that use only a single magnifier lens, for instance) are mostly restricted to systems having small fields of view (FOV), since weight and bulk increase for large FOV designs due to additional components inserted to compensate for aberrations. As an example, the recently emerging Google Glass (which has a quite thin form factor) basically consists of a small (˜1-cm diagonal) microdisplay and a simple positive lens, but has a limited FOV, beyond which aberrations become severe. On the other hand, high-end military-type displays may support an FOV approaching 150 degrees or more, but weigh as much as 5 kg or more and may contain more than 10 different lenses, most of which are present to compensate for aberrations that emerge due to the enlarged FOV. Having so many lenses is not merely a technological problem, but a fundamental one, since no single optical component can be designed to form an aberration free image of a large size microdisplay, due to the fact that the information emerging from the microdisplay quickly gets spread in space as it propagates.
Microdisplay-based HWD designs also fall short of providing the ultimate three-dimensional (3D) visual experience. These HWD designs typically provide only stereoscopic images, which invoke 3D perception essentially only through binocular disparity. Monocular cues, especially accommodation, are typically not supported, or are incorrect. Users of stereoscopic systems typically suffer from visual fatigue caused by the so-called accommodation-convergence conflict, in which eyes converge truly to the apparent position of a 3D object while accommodation is set incorrectly to the screen so as to make retinal images sharp. The fatigue is especially severe when virtual objects are closer than 50 cm.